1. Field of the Invention
The present invention relates to an optical output controlling method and an apparatus therefor, and more particularly, to an optical modulation apparatus of the type which employs a polarization beam splitter.
2. Description of the Related Art
Optical modulation apparatuses which utilize a magneto-optical material, such as MnBi or GdTbFe, have been used to magnetooptically read out an optical disk memory.
The above-described MnBi or GdTbFe is magnetized perpendicular to the surface of its film. When part of the film is heated by means of a laser beam or the like to Curie point or above or when an external magnetic field having a magnitude higher than a fixed value is applied to part of the film, the direction of magnetization of that part of the film is reversed. Hence, when the surface of the film is radiated with the light which has been polarized by a polarizer, the rotation angle (the Kerr rotation angle) of the plane of polarization of the light reflected by the portion of the film which is magnetized in the reverse direction differs from that of the plane of polarization of the light which is reflected by the peripheral portion of that portion. More specifically, the direction of polarization of the light reflected by the upwardly magnetized portion rotates from the direction of the polarization of the polarizer by +.theta.k, whereas the direction of polarization of the light reflected by the downwardly magnetized portion rotates by -.theta.k, owing to the Kerr effect, as shown in FIG. 3.
Hence, by providing an analyzer at an angle at which the plane of its polarization shifts by 45 degrees relative to that of the polarizer, as indicated by .phi. in FIG. 4, rays of light having intensity of A or B corresponding to the upward and downward magnetizations pass through the analyzer, and a hysteresis curve shown in FIG. 5 is obtained by the presence of an external magnetic field. As a result, the change in the rotation angle can be read out as a digital signal by making +.theta.k and -.theta.k correspond to "1" and "0", respectively.
In the thus-arranged conventional optical modulation apparatus, carrier to noise ratio (C/N ratio) of the signal which is read out depends on the magnitude of the Kerr effect, which is inherent in the magneto-optical material. In consequence, the Kerr effect cannot be increased when the same material is used. In other words, C/N ratio cannot be improved unless the magneto-optical material is replaced by a new one exhibiting excellent Kerr effect.